Electrode and method of making same



United States Patent O 8 Claims. (Cl. 204-38 The present invention relates to' an electrode having a base or core oftitanium or titanium with small amounts of alloying metals therein,.which core is covered with a barrier layer of titanium oxide and coated with a noble metal coating, and to a method of producing such an electrode.

This application is a continuation in-part of my application Serial No. 6,351, filed'February 3, 1960, now abandoned.

In several of my prior copending applications, there has been disclosed an electrode particularly suitable for use as an anode which has such a core or base of titanium, either substantially pure titanium or titanium with small amounts of alloying metals therein, anda coating of a noble met-a1, such as platinum, iridium, rodium, or alloys of these metals. In making such electrodes, the core of titanium is treated so as to remove any oxide coating therefrom and the noble metal-coating is formed thereon. Since such coatings are at best slightlyporous, there are places in the'coating at which the titanium core is exposed. At these pores there is formed a barrier layer of titanium oxide. This barrier layer is for-med by placing the electrode in an electrolyte and electrolytically forming the barrier layer. Alternatively, the barrier layer can be formed by a chemical and/ or a thermal treatment which makes a more stable and chemically inert barrier layer than forming the barrier layer electrolytically.

During the use of such electrodes-as'anodes in such electrolytical processes as, for example, brine electrolysis, or as anodes in cathodic protection arrangements, ionic current flow can take place through the noble metal coating and electronic current flow will take place" from the noble metal coating to the titanium core. The barrier layer over theutitanium core which is exposed through the pores in the noble metal coating will prevent ionic current flow directly to the titanium core, and will thereby prevent a chemical attack by the electrolyte, which is usually quite corrosive on the titanium of the core. In addition the noble metal coating is chemically resistant so that the titanium core is protected from chemical attack where it is coated by the noble metal.

When the noble metal coating is damaged. during electrolysis so that the bare titaniumot the core is exposed, an electrolytically formed barrier layer will be produced over the exposed area, b-utthere will be a short period of time when the titanium is exposed to attack by the electrolyte, and this is detrimental to the life of the electrode. i

I have now discovered that the titanium or titanium alloy core of such an electrode can be'entirely covered with a barrier layer of titanium oxide, and thereafter coated with a noble metal coatinggand it will still act as an electrode, particularly as an anode in electrolytic processes suchas, for example, brine electrolysis, and as an anode in cathodic protection methods. The existence of a barrier layer of titanium oxide between the noble metal coating and the titanium core or base does not. interfere with the electronic flow of current from the noble metal coating to the titanium base, while at the places where the noble metal coating is imperfectly formed or has pores in it, the barrier layer prevents ionic flow of current from the electrolyte to the titanium base.

metal.

3234,11 1h Patented Feb. 8,1966

This characteristic is generally not true of the other socalled film forming metals, i.e. the metals which have their surfaces easily oxidized by contact with the air. When these metals are coated with an oxide film, the film otters great-resistance to both ionic current flow, i.e. flow of current from an electrolyte into the metal itself, an electronic current, i.e. flow of current from a conducting metal which is in metal to metal contact with the film forming Thus, an electrode having a core of aluminum whichhas been oxidized in air or has been intentionally provided with a layer of oxide for example by anodizing, and which has been coatedwith a noble metal, offers a high resistance to the passage of electric current from the noble metal coating to the aluminum core.

The electrode according to the invention is particularly valuable in that if, during use, the noble metal coating should be damaged, for example by peeling off, 'there will be a barrier layer already formed beneath the noble metal coating which will immediately resist attack by the electrolyte, and the electrode will accordingly be preserved and itslife extended.

It is therefore an object of the present invention to provide an electrode particularly suitable for use as an anode which-has acore or base of titanium or titanium with small amounts of alloying metals therein, a barrier 1 layer of titanium'oxide covering said core or base, and a noble metal coating over the barrier layer, the noble metal being taken from the group consisting of platinum, rhodium, iridium, and alloys of these metals.

his a further'object of the present invention to provide a method of making an electrode particularly suitable for use as an anode by forming a barrier layer of titanium oxide on such a core or base, and then coating a noble have vanadium or aluminum therein in an amount up to 4% by weight, or it can have zirconium therein up to 10% by weight. These percentages of metals other than titan- ..ium will not inhibit the film forming characteristic of the titanium. Accordingly, where reference is made to a core or base of titanium in this specification, it -is to be under 'stood'th'at such a core or base includes a base of substantially pure titanium aswell as a base of titanium and an alloying metal as herein described. n t a By the term barrier layer as used to describe the oxide layer formed on the titanium core of the electrode of the present invention, is meant a layer of oxide which resists attack by the electrolyte in which the electrode is used, and at-the same time prevents passage'of current directly from the electrolyte to the titanium core but permits passage of current from the noble metal coating to the core.

In connection with the following examples, it is essential to understand that in forming the barrier layer on the titanium core by placing the core in an electrolyte as an anode and passing a current through it, the voltage which is impressed on the core must be below the breakdown, voltage for titanium in the electrolyte in'question. Otherwise, if the breakdown voltage for titanium in the particular electrolyte is exceeded, the form of oxide which will be produced is not that which produces a barrier layer, but rather is one which, when itis coated with a conducting-metal, will not permit electronic current conduction therethrough from a noble metal coating to the titanium core at the normal operating voltages of the electrodes during their use in electrolysis.

It is further necessary to understand that the breakdown voltage will vary depending on the particular elec trolyte in which the barrier layer is being formed on the titanium core, and in fact the breakdown voltage will. be further dependent on the metal of the core, should it be, for example, an alloy of titanium. For this reason,

in the examples, thevoltages at which the barrier layer is formed can vary from example to example.

invention, the voltage utilized will amount to to 60% of the breakdown voltage depending on the concentration and temperature of the electrolyte.

The method of making the electrodes according tothe invention comprises at least partially immersing a core of'titanium in an electrolyte, impressing a voltage on said core, which voltage is below the breakdown-voltage for titanium in said electrolyte, but :sufiicient 'to form a barrier layer of titanium oxide on the portion of the core which is immersed in the electrolyte, painting a solution of at least one salt of a noble metal taken from.

the group consisting of platinum, rhodium and iridium or alloys thereof in an organic volatile solvent onto the barrierilayer thus formed on said core, heatingsaid painted core to evaporate the volatile solvent,'and then. firing the ,core in an atmosphere for converting the deposited salt to metal.

The method will now be set forth'in 'detail in connection with the following examples. In these examples,

a' plurality of specimens of substantially pure titaniumplate'each 1 cm. x 2 cm. were prepared according to the conditions set forth in Table I, there being used.100. cc.:

of electrolyte in each instance, and the electricpower being supplied from an accumulator at the indicated voltages which are below the .breakdown voltages for However, for purposes of illustration, it can be mentioned that thevoltage for formingthe barrier layer herein plates were heated in airvat a temperature of 250" C. for

After: the last application of the paint and the heating in air, the painted plates'were each heated in a closedturnace in an atmosphere. of ammonia and a reducing gas,.for .ex--

ample illuminating or lighting, gas, at. a temperature of 11 minutes to evaporate and burn off the solvent.

330 for l0minutes in order to form on the plates a coating of a platinum and iridium alloy without-adversely aft footing the underlying barrier layer. :of titanium oxide. Thereafter. the plates were gradually cooled.

The thus treated specimens 1, 3,v 5 and 6 were then used as anodes in theelectrolysis of 30% brine solutions at a temperature of 60 C; and a current density of 1500 amperes per square meter, at an applied voltage of 2.8. volts. The anode'sperforrned excellently, and at a low and constant overvoltage-this current, density could "be maintained. Specimens 2 and 4 were used as anodes in-a similar chloride alkali electrolysis with excellent results.

Examples 7-1-2 Specimens -7l2 were each coated four times withta paintzwhich contained 2 gramsof rhodiumin the form. I

of a rhodium compound,'for example rhodiumchloride,

per 10 cc. of organicsolvent, for example ethanol. After" each coating the plates were heated ;in air at atempera ture of 250 C. for'l0'minutes; to evaporate, and burnoif the solvent. After the last-application,ofthe paint and the heating in :air to evaporate and .burn the solvent the painted plates were each heated in air at a temperature 1 of 650 C. for 10 minutes in order. to form :on the plates a coatingofmetallic rhodium. Thereafter the plates were gradually cooleda were mechanically very strong;

Examples 13.18

Specimens 13-18 were :each'coated twice: with a paint f l I whichcontaine'd 10% by weight platinum chloride, 20%

of. an ethereal oil (lavender) and the remainder an or v ganic diluent, for example acetone and isopropyl alcohol.

After each coating the plates were heated inc-airat 390- trtamum 111 the respective electrolytes. C. for 1 nnnute to evaporate and burn'oif the solvent, and

TABLE I i 7 Immediately after voltage is After 15 minutes I applled Resistance 7 Specimen Volt. Electrolyte C across barrier,

aqueous solution or oxide layer Intensity .ln Ionic resistance Intensity in Ionic resistance in 1 ohms rmcroamperes in ohms microamperes in ohms 4 36,000 111 57 70,250 1% NaCLQ 20" 0. 001-0. 005 6 100,000 37, 500 5% N aCl 0. 001-0. 005 6 25,000 240 52, 700 1% NaCl 0. (101-0005 4 45, 000 89 S0 50, 000 1% H450 0. 001-0. 005 4 100,000 40 100 40, 000 10% H2304. 0. 001-0. 005 4 115,000 35 130 80, 800 10% NaOH 0. 001-0. 005 6 50, 000 46, 200 1% NaCll. 0. 001-0. 005 4 45, 000 89 75 53, 300 36% NaCL. 0. 001-0. 005 4 50, 000 80 45 88, 900 5% NaCl.-- 0. 001-0. 005. 4 100, 000 40 100 40, 000 10% H 0. 001-0. 005 4 90, 000 44 90 44, 500 5% NaO 0. 001-0. 005 4 34, 000 118 20 200, 000 10% tartaric a 0. 001-0. 005 4 55,000 73 85 7,100 5% NaC 0. 001-0. 005 6 100, 000 60 42, 800 36% NaCL. 0. 001-0. 005 4 15, 000 267 85 47, 100 1 0 NaCl 0. 001-0. 005 p 4 100,000 40 70 57, 200 5% Has 4. 0. 001-0. 005 4 60, 000 67 85 47, 100' 1% NaOH 0. 001-0. 005 4 300, 000 13 15 267, 000 10% oxalic acid I 0. 001-0. 005 4 60, 000 67 55 72, 750 1% N aCl 0. 001-0. 005 4 75, 000 53 55 72, 750 5% NaCl--- 80- 0.'00l0. 005 a Examples 1-6 Specimens.16 were each coated four times with a 70 paint which contained 1 gram of platinum in the form.

of a platinum salt, for example platinum tetraiodide, and v 0. 03 gram of iridium in the form of an iridium salt, for 7 example iriditun tetrachloride, per 10 cc. of an organicv to deposit a layer of metallic platinum on the barrier" layer on the titanium. cores;

solvent, for. exampleethanol. After each coating the -water, and proved very satisfactory.

Thereafter the platinum coated titaniu'm'electrodes were heated in air at 350 C. for 48 Example 19 A plate of substantially pure titanium was partly coated with platinum, and was at least partially immersed as an anode in an electrolyte which was an aqueous solution having 5% NaCl at a temperature of 70 C. so that at least part of the platinum coated portion of the electrode was in the electrolyte. A voltage of 4 volts was applied to the electrode to form a barrier layer on the uncoated portions of the electrode which were immersed in the electrolyte. Immediately after the application of the voltage, the current intensity was 1,000,000 microamperes and the ionic resistance was 4 ohms. After the voltage had been applied for minutes, the current intensity was still1,000,000rnicroamperes and the ionic resistance was still 4 ohms. A barrier layer was formed on the uncoated parts of the electrode-which had a resistance as measured A plate of titanium metal is thoroughly degreased by rinsing it with e.g. petrol or carbon tetrachloride. The plate is dried and placed as anode between two graphite cathodes in a solution of 95 parts by volume of concentrated phosphoric acid (98% or more) and 5 parts by volume of concentrated nitric acid. The voltage between the titanium anode and the graphite cathodes is gradually raised to 100 volts, a barrier layer of titanium oxide forming which also after the termination of the electrolysis remains of excellent quality.

On to the plate of titanium provided with said electrolytically formed barrier layer a homogeneous mixture is sprayed which is composed of 100 parts by volume of absolute ethanol,

10 parts by volume of rhodium trichloride,

2 parts by volume of colored Venetian turpentine,

5 parts by volume of hydrazine-monohydrochloride.

When from the color it appears that the entire plate is covered by the mixture, the plate is dried and the whole is heated in an open flame to a temperature of at most 700 C. A rhodium coating of about 2 microns will form.

The electrode thus manufactured is found to be very satisfactory when used as anode for carrying out electro lyses of all kinds of electrolytes, more particularly also of baths containing chloride, with the exception only of electrolyses evolving fluorine. The allowable current density is 70 amperes per square decimeter or higher.

Instead of the mixturementioned above it is also possible to apply by means of a brush mixture of 100 parts by volume of absolute ethanol, 10 parts by volume of platinum tetraiodide, 2 parts by volume of rhodium trichloride, 1 part by volume of lavender oil,

3 parts by volume of colophonium resin, 5 parts by volume of hydrazine.

By the method according to the invention there has been produced an electrode in which there is a barrier layer completely covering the titanium of the base, so that where there are pores or breaks in the noble metal coating on the electrode, the titanium of the base is protected by the barrier layer. Should the coating peel or otherwise be removed, all that is exposed is the barrier layer, the titanium of the core being protected from attack by the electrolyte in which the electrode is immersed.

In addition, it is possible with this type of electrode to provide the electrode with a fresh coating of noble metal a substantially unlimited number of times, if desired. The barrier layer remains intact once it has been formed, so that it is not necessary to pickle the electrode before applying a new noble metal coating, as has heretofore been done with this type of electrode. Moreover, there will be no loss of the core material during recoating, since the noble metal is simply added to the barrier layer covered core.

A further advantage of the electrodes according to the invention is that the adherence of the coating of noble metal is enhanced because the surface of the oxide barrier layer is rougher than the surface of the bare titanium metal.

It is thought that the invention and its advantages will be understood from the foregoing description and it is apparent that various changes may be made in the method without departing from the spirit and scope of the invention or sacrificing its material advantages, the forms of the method hereinbefore described and set forth in the examples being merely preferred embodiments thereof.

I claim:

1. An electrode, comprising a core of a metal taken from the group consisting of 'titanium and an alloy of titanium and small amounts of alloying metals, a high ionic resistance, low electronic resistance barrier layer of titanium oxide covering at least a part of said core, and a noble metal coating over said barrier layer, the noble metal being selected from the group consisting of platnium, rhodium, iridium, and alloys thereof, said barrier layer being electrolytically deposited on said core by impressing thereon a voltage below the breakdown voltage of the metal of the core in the electrolyte used for the formation of the barrier layer.

2. An electrode, comprising a core of a metal taken from the group consisting of titanium and an alloy of titanium and small amounts of alloying metals, a high ionic resistance, low electronic resistance barrier layer of titanium oxide covering at least part of said core, and a noble metal coating over said barrier layer, the noble metal being selected from the group consisting of platinum, rhodium, iridium, and alloys thereof, said barrier layer being electrolytically deposited on said core by im pressing thereon a voltage below the breakdown voltage of the metal of the core in the electrolyte used for the formation of the barrier layer, and the noble metal coating being deposited on the barrier layer by painting onto the barrier layer a solution containing a compound of the noble metal and evaporating the solvent.

3. An electrode as claimed in claim 2 in which the alloying metals are taken from the group consisting of up to 4% vanadium, up to 4% aluminum, and up to 10% zirconium.

4. An electrode, comprising a core of a metal taken from the group consisting of titanium and an alloy of titanium and small amounts of alloying metals, a barrier layer of titanium oxide covering at least a part of said core, and a noble metal coating over said barrier layer, the noble metal being selected from the group consisting of platinum, rhodium, iridium, and alloys thereof, said barrier layer having an electronic resistance in the range of about 0.001 to 0.005 ohm and an ionic resistance of at least about 35,000 ohms which is formed by immersing the core in .a sodium chloride electrolyte for about 15 minutes with an impressed voltage of about 4 volts.

5. A method of making an electrode, comprising the steps of immersing at least a part of a core of a metal taken from the group consisting of titanium and an alloy of titanium and small amounts of alloying metals in an electrolyte as an anode, impressing a voltage on said core, which voltage is below the breakdown voltage for titanium in said electrolyte, to thereby form a high ionic resistance, low electronic resistance barrier layer of titanium oxide on the immersed part of said core, and then coating over said barrier layer a noble metal taken from the group consisting of platinum, rhodium, iridium, and alloys thereof.

6. A method of making an electrode, comprising the steps of immersing at least a part of a core of a metal taken from the group consisting of titanium and an alloy 7 t of titaniumand small amounts of alloying metals in an electrolyte as an anode, impressing a voltage .on said core,

which voltage is below the breakdown voltage for titani-' um in said electrolyte, to thereby form a high ionic resistance, low electronic resistance barrier layer of titanium oxide on the immersed part of said core, and then paintinggonto said barrier layer covered core a solution containing a compound of a noble metal taken from-the group consisting of platinum, rhodium, iridium, and alloys thereof, andheating the painted core for evaporating the solventand converting the compound to a metallic coating while retaining the barrier layer undisturbed. V

7. A method as claimed in claim 6 in which the painted core is heated in air for evaporating and. burning the solvent, and then is heated in a reducing atmosphere for reducing thecornpound of the noble metal while leaving the barrier layer unaifected.

8. A method of making an electrode, comprising the steps of partly coating a core of a metal taken'fromthe group consisting of titanium and an alloy of titaniumand small amounts of alloying metals with a noble metal,

immersing at least a part of the core including a part of Y the noble metal coated portion in an-electrolyte as an anode, impressing a voltage onsaid core, which voltage is below the breakdown voltageior, titanium in ,said i electrolyte, for forming a high ionic resistance, lowelecrhodium, iridium, and alloys thereof.

References Cited by the Examiner 10 ,UNITED STATES PATENTS 2,955,999 10/1960 Tirrell 204-290 2,965,551; 12/1960 'Richa'ud 204-32 2,991,234 7/1961 Andrus -204-3s 3,085,052 4/1963 Sibert 204--38 3,096,272v 7/1963 Beer 204-290 3,103,484 9/1963 Messner 204-290 OTHER REFERENCES 20 Chemical Age, page 9, January 3,4959,

WINSTON r A. DoUGLAsrimar Examiner.

' JOHN H. MACKQExaminrQ 

1. AN ELECTRODE, COMPRISING A CORE OF A METAL TAKEN FROM THE GROUP CONSISTING OF TITANIUM AND AN ALLOY OF TITANIUM AND SMALL AMOJNTS OF ALLOYING METALS, A HIGH IONIC RESISTANCE, LOW ELECTRONIC RESISTANCE BARRIER LAYER OF TITANIUM OXIDE COVERING AT LEAST A PART OF SAID CORE, AND A NOBLE METAL COATING OVER SAID BARRIER LAYER, THE NOBLE METAL BEING SELECTED FROM THE GROUP CONSISTING OF PLATNIUM, RHODIUM, IRIDIUM, AND ALLOYS THEREOF, SAID BARRIER LAYER BEING ELECTROLYTICALLY DEPOSITED ON SAID CORE BY IMPRESSING THEREON A VOLTAGE BELOW THE BREAKDOWN VOLTAGE OF THE METAL OF THE CORE IN THE ELECTROLYTE USED FOR THE FORMATION OF THE BARRIER LAYER. 